Field of the Invention
The present invention relates to a thin film piezoresonator which, in particular, can be used as a filter in BAW components.
In BAW (bulk acoustic wave) components, a piezoelectric effect which occurs in specific materials is used to generate a mechanical stress in the material through the use of applied electrical voltages. Conversely, mechanical deformations in a piezoelectric material produce electrical voltages. Layers of piezoelectric material, referred to below as piezo layers, can be excited into acoustic oscillations in the range of GHz frequencies if they have a suitable thickness in the range of a few micrometers and are disposed between electrode layers. If the piezo layer is insulated acoustically from the environment, standing waves can be produced, and the component can be operated at resonance. One application of that principle is known from quartz clocks, crystal filters and BAW filters. Acoustic insulation can be provided by applying the piezo layer as a thin membrane, so that it is surrounded all around by air. Another possibility is to use acoustic Bragg reflectors as a stack of layers with a thickness of one quarter wavelength. The individual layers of the Bragg reflector in that case are formed of materials of different acoustic impedances. The thickness of the piezo layer substantially determines the resonant frequency. Therefore, fluctuations in layer thickness during a manufacturing process have a considerable effect on the resonant properties of a BAW component. European Patent Application EP 0 865 157 A2, corresponding to U.S. Pat. No. 5,872,493, describes a BAW filter with two piezoelectric layers of similar materials and thicknesses, disposed as a SCF (stacked crystal filter).
In the case of a thin film piezoresonator, the resonant frequency can be retuned by a structured layer being applied to the piezo layer. Instead of structuring a single layer, a series of a plurality of unstructured layers of different thicknesses can be applied, in order to suppress one or more resonant frequencies. The resonant frequency can be shifted through the use of connected capacitances or an applied D.C. voltage. However, the tuning range is very low in that case and barely adequate for practical purposes. Even when very high D.C. voltages are applied, the relative length expansion of typical materials used for piezo layers (e.g. ZnO, AlN, PbZrTi) is less than 0.1%. Materials with more efficient length expansion, such as the polymer PVDF, are unsuitable in the range of high frequencies.
It is accordingly an object of the invention to provide a piezoresonator, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which can be tuned over a wide range in the range of radio frequencies.
With the foregoing and other objects in view there is provided, in accordance with the invention, a thin film piezoresonator, comprising first, second and third electrode layers. A piezo layer is disposed between the first and second electrode layers and an electroactive or electrostrictive layer is disposed between the second and third electrode layers.
The piezoresonator according to the invention uses a layer of at least predominantly electrostrictive electroactive materials which is referred to below as an electroactive layer, in order to tune the resonant frequency of a piezoelectric BAW resonator. An electroactive material is understood to be a material which exhibits the piezoelectric effect or its converse, electrostriction, which is formed in a mechanical deformation of the electrostrictive material occurring when an electrical voltage is applied. The effect of electrostriction is used, for example, in a quartz crystal which is set into resonant oscillations by applying an alternating electrical voltage. Conversely, the piezoelectric effect permits an electrical voltage to be tapped off if the piezoelectric material is deformed suitably. There are purely electrostrictive materials, in which this converse effect does not occur or occurs to only a very slight extent. Therefore, such materials do not generate any electrical voltage in the event of a deformation. This property is possessed by some nonpolar ceramics, in particular PMN (lead magnesium niobate) and a series of polymers (for example polyvinylidene fluoride hexafluoropropylene [P(VF2-HFP)]). Such materials, which are electrostrictive in this sense, are preferably used as the electroactive layer in the piezoresonator according to the invention. The effect of electrostriction is nonlinear to a great extent. The mechanical deformation depends on the square of the existing electrical field. In the piezoresonator according to the invention, the piezo layer is connected to an electroactive layer, preferably one made of a purely electrostrictive material, which contracts in the vertical direction, that is to say is thinned, when a D.C. voltage is applied. This changes the resonant frequency of the layer stack. The piezo layer is disposed between electrode layers. The electroactive layer is disposed on one of these electrodes on the side facing away from the piezo layer, and has a third electrode layer for the application of the D.C. voltage. Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a piezoresonator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.